Description of Related Art
Hydrocarbons within a subterranean formation are normally removed from the formation by primary recovery methods which employ naturally occurring water or gas pressure found within the formation to carry hydrocarbons to the earth's surface. When naturally occurring formation pressure is insufficient or formation pressure has been depleted by hydrocarbon production, secondary recovery methods must be used to produce hydrocarbons from the formation.
One type of secondary recovery is the use of soluble or miscible phase displacement techniques wherein aqueous solutions are injected into wells which are in fluid communication with hydrocarbon bearing subterranean formations. The solutions are soluble or miscible with the hydrocarbons and cause the hydrocarbons to be displaced from the pores within the formation. Once displaced, the hydrocarbons are pushed toward production wells by drive fluids. A common problem with these types of hydrocarbon recovery methods are that aqueous solutions often have poor injection profiles due to low solution viscosity. Low viscosity results in fingering and a reduction of hydrocarbon recovery.
The ability of a solution to finger through a formation is dependent on fluid "mobility". A laboratory method for obtaining a measure of the relative mobility of solutions is described in the American Institute of Mining Engineers' paper "Factors Influencing Mobility Control by Polymer Solution" by R. R. Jennings, J. H. Rogers, and T. J. West, SPE Paper No. 2867 prepared for the Ninth Biennial Production Techniques Symposium, held in Wichita Falls, Tex. May 14-15, 1970. In this method, the solution to be tested flows through screens and the ratio of the time required for the test solution to flow through the "screen viscometer" divided by the time required for a standard solvent (water) to flow through the viscometer is termed the "screen factor". Screen factors indicate the effectiveness of a solution's mobility control. The greater the screen factor the better the mobility control.
Generally, mobility control in a waterflood is achieved by addition of polymers to increase the viscosity of the aqueous solution. The increased viscosity decreases the channeling of the aqueous solution through the formation, and thereby improves the efficiency of oil recovery from the production well. However, the greater the screen factor and mobility control the more difficult an aqueous solution is to inject due to increased solution viscosity. Additionally, increasing solution viscosity often causes plugging within the subterranean formation due to coagulated polymeric materials blocking the pores within the formation. To mitigate the problem of formation plugging, it is common in the industry to filter the aqueous polymer solution to remove the coagulated polymers prior to injection into the formation.
The use of aqueous solutions of polymeric materials injected into a subterranean hydrocarbon-bearing formations via injection wells to enhance recovery of hydrocarbons from production wells is well known in the art. Also well known is the addition of various organic chemicals to the aqueous polymeric solutions to achieve either changes in viscosity or changes in filterability. U.S. Pat. No. 4,212,748 to Ferrell et al. discloses a method for improving polymer augmented waterflood filterability by addition of sulfated ethoxylated alcohol and an ethoxylated alcohol to the polymer solution. The method enhances filterability of the aqueous solution so that it possesses improved injectivity characteristics when used as a formation waterflood. U.S. Pat. No. 4,676,316 to Mitchell discloses a process for preinjecting an aqueous solution of water-soluble polymer and an anionic surfactant into a hydrocarbon bearing formation followed by injection of a gas. The surfactant improves both mobility and profile control of the aqueous polymer solution and thereby improves hydrocarbon recovery.
The Ferrell patent recognized that filterability of flood solutions are improved by adding both sulfated ethoxylated alcohol and ethoxylated alcohols to the waterflood. The Mitchell patent recognized that nonionic surfactants such as alcohol ethoxylates improved mobility and profile control in a gas flooding process. However, neither these patents have recognized that when ethoxylated alcohols are added to aqueous solutions of partially hydrolyzed polyacrylamides that the solution screen factor increases (i.e. solution mobility control improves, mobility is decreased and fingering is decreased) at the same time that viscosity decreases (i.e. filterability improves). Accordingly, it is an object of this invention to present a method to reduce fingering within a waterflood and improve injectivity by simultaneously increasing the screen factor and decreasing viscosity of an aqueous PHPA solution. The improved injectivity and screen factors result in improved recovery of hydrocarbons from the formation by reducing the difficulty of injecting the aqueous solution into the formation and improving the mobility control of the aqueous solution as it displaces hydrocarbons toward the production well.